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Old 04-02-2014, 07:21 PM   #15
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You should be able to get it weighed at a CAT scale if you straddle two of the weighing pads.
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Old 06-01-2014, 12:22 PM   #16
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When we're talking tire pressures here, is it per axle regardless of the number of tires on the axle? e.g. if my vehicle axle weight in the rear dictates 80 pounds of tire pressure, can I run softer air pressures because I have four tires on that axle? That is not considered dual is it? The manufacturer chart that says 'single' and 'dual' for Michelin XRV's so if I have 4 tires on the rear axle and 2 tires up front on the front axle do I use the chart 'dual' for the rear and 'single' up front?

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Old 06-01-2014, 12:40 PM   #17
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I just got weighed at a truck stop. I pulled the front axle onto the scale about 1/3 the length of the motorhome and weighed in at 6,100 pounds. I then pulled the entire length of the motorhome onto the scale and weighed in at 18,500 pounds. Subtracting the front 6100 from the total weight gave me 12,400 pounds for the rear. This was fully loaded with water and gasoline and all of our gear giving me the 'travel trim' weight. I am wondering how accurate weighing it that way is. There wasn't any other way to do it.

According to those weights at the Michelin web site, my Michelin XRV's (245/70R 19.5's) call for 75 psi for single axle front (6080 lbs.) and 85 psi for dual axle rear (12460 lbs.) but not sure if having one axle in the rear with 4 tires on it is what they consider 'dual' on their chart?

http://www.michelinrvtires.com/miche...ion-tables.jsp

Thoughts?

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Old 06-02-2014, 03:15 AM   #18
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I just got weighed at a truck stop. I pulled the front axle onto the scale about 1/3 the length of the motorhome and weighed in at 6,100 pounds. I then pulled the entire length of the motorhome onto the scale and weighed in at 18,500 pounds. Subtracting the front 6100 from the total weight gave me 12,400 pounds for the rear. This was fully loaded with water and gasoline and all of our gear giving me the 'travel trim' weight. I am wondering how accurate weighing it that way is. There wasn't any other way to do it.

According to those weights at the Michelin web site, my Michelin XRV's (245/70R 19.5's) call for 75 psi for single axle front (6080 lbs.) and 85 psi for dual axle rear (12460 lbs.) but not sure if having one axle in the rear with 4 tires on it is what they consider 'dual' on their chart?

Michelin North America RV Load & Inflation Tables

Thoughts?

Chuck
A bit confusing list that of Michelin, they give per tire for single load and per 2 tires for dual load.
Then I searched back the pressures and loadcapacity's you gave and saw you looked in the list of 225/70R19.5 instead of sises 245/70R19.5 .
Then the list for F load in right sise? is shorter then the H load , wonder why they do it.
then I also see some other strange differences , for instance the 90psi loadcapacity of F load is lower then that of H load wich is to laws of nature not expected. At 80 psi its the way you should expect so for stiffer H load lower loadcapacity then for less stiff F load.

Wanted to put your data in my motorhome calculator , because you gave all the needed details I thought, but stopped because I dont know if you have F- or H- load , What is written on sidewall counts , mostly something like this " maximum load xxxx lbs AT yyy psi ( cold)".
When you are looking also search for the speedcode ( letter J for 100km/62m/h to Q for 160km/99m/h, probably N for 140km/86m/h) .

In Europe where I live on such large tires on trucks always second loadindex is given for lower speed, try to find that too , see if in America Michelin uses that system too.
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Old 06-02-2014, 08:33 AM   #19
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You are right. I am looking at 225/70R19.5 table. I looked at my tires and they are "LRF" or load range "F". On the side of the tire it reads "Max Load Single 1350kg (4,080 lbs) at 95 psi cold" and "Max Load Dual 1750kg (3,860 lbs) at 95 psi cold". According to their table, I am off the scale at 6100 pounds. Please tell me that can't be right!
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Old 06-03-2014, 02:50 AM   #20
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I asume the short list is because they asume more is not used for Motorhomes, and this F -load is meanth mainly for Motorhomes.
But this does not mean that for laws of nature you are not allowed to go lower. If calculated with the ideal formula, that takes care of the same deflection over the whole ranche, you could even go as low as zero pressure if the load is low enaugh, is my conclusion in time.

In this exlamples map of my motorhome-calculator map I have put the spreadsheet with your username. Cant make a picture to show here on this old computer I am working on now ( windows XP).
https://onedrive.live.com/?cid=A526E...E092E6DC%21836
You dont need to know how Excell works , only have to have it on your computer. Thougt I already described how to download.

Then going from your tires maximum load and reference-pressure ( corrected the KG for single) it gave front 77 psi and rear 84 psi with both 10% reserve added to the given loads. Loadivision F33% R 67% , once read that front is adviced to keep above 30% so OK.
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Old 06-03-2014, 05:18 AM   #21
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Originally Posted by chucksel View Post
You are right. I am looking at 225/70R19.5 table. I looked at my tires and they are "LRF" or load range "F". On the side of the tire it reads "Max Load Single 1350kg (4,080 lbs) at 95 psi cold" and "Max Load Dual 1750kg (3,860 lbs) at 95 psi cold". According to their table, I am off the scale at 6100 pounds. Please tell me that can't be right!
Your math is a bit off. Front axle has 6100 Lbs. on 2 tires which splits the load to 3050 Lbs. per tire. The rear at 12,400 Lbs. is spread over two pairs at 6200 per pair. As such the front should have 80 PSI and the rear 85 PSI. I rounded up on weight for Michelin's chart which will give you a bit of extra safety margin.

Air up to what the chart calls for as dependent of axle tire configuration be it single or dual. Do not double a single or half a dual, stick to what the chart states. If you have any doubt on the coach weight you have two choices. Air up to the sidewall maximum pressure or use the builder's label in the coach which is usually based on maximum axle loading AND air pressure to take that weight for the tire size recommendation for the model year.
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Old 06-04-2014, 01:40 AM   #22
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I assumed Chucksel meant with 6100 being off the scale that he already devided it by 2 and that 3050 was lower then the lowest given loadcapacity of that short list for F load. "But then again I can be wrong too" .

then my new idea about the lists of Michelin.
I would not be surprised if the H load XZE was practically the same tire as the XRV F-load, yust with another stamp on it.
I make that of the fact that for 95 psi F gives 4080lbs/1850 kg and the H gives 4100 lbs/1860kg. Standard is that an F load gives more loadcapacity for the same pressure.
The small difference is to fit it into the Loadindex system.

Then the RV F load is calculated with the standard TRA formula with power 0.7 and the H load calculated with standard ETRTO power of 0.8 , I calculated back.
All tires worldwide are calculated in their maximum load with the same formula using international standard of KG and kPa ( kiloPascal 100kPa=1 bar=14.5 psi), and then for the American market conversed to LBS and Psi.
In that proces double roundings are done, and the pressure/loadcapacity lists are also first made in kPa/KG then rounded down to 5 or 10 kg, then conversed to PSI/LBS where PSI is also rounded and LBS again rounded to 5 or 10 lbs.
so in this proces generates some inacuracy .

My quess is that the F load is in fact about the same construction as the H load but to fit in the stif american system with 0.7 power, Michelin ( European tyre maker French) compensated that wrong calculation by giving lower maximum load and so the 80 psi still gives with wrong 0.7 calculation a still save deflection of tire.
If they went lower the calculation leads to to much deflection, so they give that short list.

If you now calculate with my made spreadsheets , I use a to my conclusion save calculation ( even saver then the EUR 0.8 power) that gives the same deflection over the whole range, and so you can go lower in the pressure with save deflection.
Then even some reserve build in so the 77/84(cold) I gave are filling pressures. If on the road you measure for instance 70/75 psi(cold) , with some luck ( equally devided R/L, lower speed then calculated for, acurate devices) you are still not giving damage to the tires.
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Old 06-04-2014, 07:14 AM   #23
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Tire Pressure

Thanks for all the math and head scratching.

So it sounds to me that the formula is weight over the axle divided by number of tires on the axle so 6100 on the front axle divided by 2 tires equals 3050 and Michelin chart for 245/70R19.5 LRF XRV starts at 80 psi for 3640 lb per tire (so maybe can go 76 or 78 in front)? And for the back, 4 tires on the axle with 12,500 lb weight is 3125 lb per tire or 6250 per side. How come dual chart only starts out at 6830 at 80psi? This is pretty confusing because off to the left it says max 3860 (per tire) at 95 psi?

I am getting ready for a long trip on Thursday and these tires are only months old with 1,000 miles on them. I don't want to lose a tire (or a LIFE) because of a blowout !

Thanks everyone!
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Old 06-04-2014, 07:50 AM   #24
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That is what I thought strange about the Michelin chart.
In America they only give per tire.
And in Europe they give the lists per axle single and axle dual , so single 2 times tires maximum load for single and dual 4 times maximum load for dual wich is 7.5 % lower then maxload single so maxloaddual = maxloadsingle x 0.925.
American sometimes uses 9% lower for dual then single , but that is a mater of opinion about what is needed between the 2 systems.

so giving for 1 tire for single and for 2 tires for dual is the strange unorthodox thing in that list of Michelin. The EUR press/loadcapacity- lists are that way so you dont have to do the division by the number of tires per axle yourselfes, and giving per tire and 2 tires needs still your calculation. If you had weighed per wheel( pair) so 4 point weighing this list is handy because then you dont have to devide by 2 for the duals.

The 6830 for dual at 80 psi is calculated back from 2 times dual maxload of 3860 at 95 psi= 7720 lbs. For single the maxload is 4080 lbs a tire
3860/4080=0.946 and not 0.925 , but that is because tires are first calculated in maxload for single , then from that 0.925x single =dual . Then both are rounded down to the first lower Loadindex step.

Lets use this as example to show what is done, in kg because thats the original calculation. so S1850 kg D1750 kg
1750/0,925=1891 kg wich is the original calculated single maxload at the lowest. This 1891 kg is yust not enaugh to call it a 130 Loadindex of 1900 kg maxload So is rounded down to LI 129 with maxload of 1850.
So the calculated single maxload is between 1891 and 1899 kg( 1900-1) and from that the maxload of dual is calculated. The original calculated single is lower then LI130 so 1900kg so1899 x 0,925 =1760 kg dual maxload
So maxload dual between 1750 and 1760 kg calculated and so rounded down to LI 127 with 1750 kg/3860 lbs maxload.

the lower maxload of dual is done as far as I concluded to cover if one of the duals gets more weight on them then the others in use. This can happen for instance when driving in a truck track ( can give one tire more deflection) or the sideward curve of the road( marginal effect)

This al for who interests it , but also to make you understand the system , so you yourself can make logical decicions.
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Old 06-05-2014, 05:10 AM   #25
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Chucksel don't over work this. Don't divide the rear by four. Stick to the DUAL inflation listing. Round UP to the pressure closest to your weight. If you're off the bottom don't try and to noodle a new number. Based on your listed weight run 80 PSI in all tires.

For my coach - normal load weight for two adults is 15,900 on a 18,000 F53 chassis. Front weight 5620 (2810 per side), rear 10280 (5140 per dual set). My tires are Kelly KSR 245/70R 19.5 and for my weight I'm off the bottom of the inflation chart just like you are. Coach builder recommends 80 PSI all around which works out to give my tires a capacity equal to the axle capacity. I'm over inflated for the load which may effect the ride quality but I don't have to worry about exceeding the tire(s) load capacity for the inflation pressure I run with.
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Old 06-19-2014, 08:52 AM   #26
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I've been running 80 in front and 85 in the rear on my trip fully loaded with water and fuel and have gone up to 75 mph without an issue with the tires. I have been cruising regularly at 65 mph and all is well.

Happy travels, all!

Chuck
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